1. Field of the Invention
The present invention relates to a fine particle measurement system for measuring the amount of fine particles such as soot contained in a gas.
2. Description of the Related Art
Conventionally, a fine particle measurement system is known for measuring  the amount of fine particles such as soot contained in exhaust gas from an internal combustion engine such as a diesel engine. This fine particle measurement system generates ions by corona discharge to electrically charge the fine particles in the exhaust gas by the ions thus generated, traps the ions that are not used for electrically charging the fine particles, and based on the trapped ion amount (or based on the amount of the ions used to electrically charge the fine particles, and that are not trapped), and measures the amount of the fine particles in the exhaust gas. The amount of the trapped ions correlates with the amount of the ions used for electrically charging, while the amount of the ions used for electrically charging correlates to the amount of the fine particles in the exhaust gas, so that this fine particle measurement system is capable of measuring the amount of the fine particles in the exhaust gas based on the amount of the trapped ions.
Concerning this fine particle measurement system, known in the art is an ion-source power supply circuit for generating ions, a source power supply circuit casing that surrounds the ion-source power supply circuit, and a signal current detection circuit for detecting a signal current which correlates to the amount of trapped ions. These components are electromagnetically shielded by a surrounding outer circuit casing (Patent Literature 1). In addition, by using a separation electrode for generating a repulsive force on ions that are not used for electrically charging fine particles, and the separation electrode is disposed so as to pass through a nozzle component member including a nozzle for injecting the ions, where the separation electrode and the nozzle component member are insulated by a ceramic member (Patent Literature 2).
[Patent Literature 1] JP-A-2012-220423
[Patent Literature 2] JP-A-2012-194078
3. Problems to be Solved by the Invention
However, as for the fine particle measurement systems disclosed in the above-described related art, there is still room for improvement in terms of accuracy in detecting the amount of fine particles in exhaust gas. In general, the current corresponding to the amount of ions that are used for electrically charging the fine particles is very small, which makes it difficult to detect a current value with accuracy. For example, an unintended slight current (leakage current) occasionally flows in a circuit of the fine particle measurement system. This results from deteriorated insulation performance and the like, which might be caused by an applied high voltage for discharge purposes or humidity of an insulation members used for the fine particle measurement system. When the leakage current is generated, an error occurs in a current value that is detected as the current corresponding to the amount of ions that are used for electrically charging the fine particles.
On conceivable example of the fine particle measurement system is a fine particle measurement system that includes a power supply circuit on a primary side of an isolation transformer, an ion generating unit and an ion trapping unit on a secondary side of the isolation transformer, and a current measurement circuit that is disposed between the primary side and the secondary side. The current measurement circuit supplies the secondary side with a current corresponding to the amount of ions that are used for electrically charging the fine particles as a compensating current, and detects the current value of the compensating current. With this configuration, it is possible to detect the amount of the fine particles in exhaust gas based on the current value of the compensating current detected by the current measurement circuit. However, in this fine particle measurement system, when a leakage current runs from the primary side to the secondary side in an area other than the current measurement circuit, an error occurs between the current value of the compensating current and the current value of the current corresponding to the amount of ions that are used for electrically charging the fine particles. A problem thereby arises in that the amount of the fine particles in exhaust gas cannot be detected with accuracy.